This invention relates to a method of making a semiconductor device such as a double diffused MOS transistor.
According to a prior art method of making an N-channel double diffused MOS transistor, as shown in FIGS. 6A-6D (summarily referred to as FIG. 6), a gate oxide film 2 is formed first on an N-type silicon substrate 1 and a gate electrode 3 made of poly-crystalline silicon is then formed on this gate oxide film 2. A doping process is carried out thereafter, as shown in FIG. 6A, by using this gate electrode 3 as a mask and implanting P-type impurities such as boron by an ion implantation method. In the figure, the implanted dopant is indicated by "x." Next, the impurities are driven in by an annealing process, as shown in FIG. 6B, and P-type diffusion areas 4 and 5 are formed by using the gate electrode 3 as a mask for self-alignment. There are usually large differences in the dimension of such diffusion areas 4 and 5 in the (vertical) direction of depth and the (horizontal) direction of extension. The depth x and the horizontal extension y are usually in an approximate relationship of x:y=1:0.7-0.8. Next, another doping process is carried out again by using the gate electrode 3 as a mask and implanting N-type impurities such as arsenic or phosphorus by an ion implantation method, as shown in FIG. 6C. Next, these impurities are driven in, as shown in FIG. 6D, by an annealing process. N-type diffusion areas are thus formed inside the P-type diffusion areas 4 and 5 to become sources 6 and 7 by using the gate electrode 3 as a mask for self-alignment. At the same time, the P-type diffusion areas 4 and 5 become channel regions. The difference in the distance between the P-type diffusion area 4 (or 5) and the source 6 (or 7), thus formed, becomes the channel length L.
In order to improve the characteristics of such a MOS transistor, it is required to reduce its threshold voltage. The threshold voltage can generally be made smaller by reducing the impurity concentration in the channel region and also by making the gate oxide film thinner. If the impurity concentration in the channel regions is made smaller, however, the so-called punch-through phenomenon is likely to occur whereby adjacent hollow regions extend and strike each other, and this will have the consequence of adversely affecting the voltage-resistance of the transistor. In order to prevent the voltage-resistance from becoming adversely affected by the punch-through phenomenon caused by a drop in the impurity concentration in the channel region, one may think of making the channel length longer but the process of driving in ions will have to be carried out at a higher temperature and for a longer period of time in order to increase the channel length. If the drive-in process is carried out at a high temperature and for a longer period of time, however, the impurities which have been implanted into the polysilicon gate electrode at a high concentration may pass through the gate oxide film and reach the side of the silicon substrate. Such penetration of the impurities to the side of the silicon substrate has the effect of adversely affecting the general characteristics of the transistor element itself.
In summary, the drive-in process for channel diffusion must be carried out at a high temperature for a longer period of time, while the impurity concentration in the channel regions is kept low, in order to make the threshold voltage lower without lowering the voltage-resistance caused by punch-through, but such a process leads to the problem of impurities of the polysilicon reaching the silicon substrate as a result of the drive-in process at a high temperature over a long period of time.